Matched electric and magnetic field homogeneity for optimum resolution cycloidal mass spectrometers



April 8, 1969 H. W. BROWN MATCHED ELECTRIC AND MAGNETIC FIELD HOMOGENEITY FOR OPTIMUM ERS , RESOLUTION CYCL'OIDAL MASS SPECTROMET Filed Feb. 21, 196 Sheet 1 of g f ry-1 r i n N: PUMP v 2 V r H i GAS L SOURCE W I J \\V ,5 t AMPLIFIER /-=l Y I RECORDER d6 @1 T l H 1 2 T \d rfl-- SCAN mg v GENERATOR V $4 r a I I J I I a I INVENTOR- HARMON v1. aRovm April 8, 1969 MATOHED ELECTRIC AND MAGNETIC FIELD HOMOGENEITY FOR OPTIMUM Filed Feb. 21, 1966 H. W. BROWN RESOLUTION CYCLOIDAL MASS SPECTROMETERS Sheet FI'G.2 v

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HAR 0N W. BROWN United States Patent US. Cl. 25041.9 8 Claims ABSTRACT OF THE DISCLOSURE A high resolution cycloidal mass spectrometer is disclosed. The spectrometer includes an array of ring-shaped electrodes, separately energized with different electrical potentials for producing an ion focusing electric field at right angles to a D.C. magnetic field in which the array of electrodes is to be immersed in use. Gas to be analyzed is ionized in an ion source and the ions are projected into the analyzing region containing the crossed electric and magnetic fields. Under the influence of the crossed electric and magnetic fields the ions are caused to traverse a cycloidal trajectory with ions of a certain chargeto mass ratio being focused at an ion detector. The magnetic or electric field intensity is scanned to produce an output mass spectrum. It has been found that a certain minimum number of electric field producing electrodes may be utilized to obtain a certain maximum obtainable mass resolution when the electrodes are dimensioned according to the formula;

where W is the width of the open space in the ring-shaped electrodes taken in a direction normal to the ion trajecto- V ries, S is the spacing between the midplanes 'of adjacent electrodes taken in the direction along the axis of the array of electrodes and at right angles to the direction along which W is measured, and wherein R is the mass resolution of the derived output spectrum.

Heretofore cycloidal mass spectrometers of the type shown and described in US. Patent 2,221,467, issued Nov. 12, 1940, have been built. Certain of these prior spectrometers have exhibited high resolution of the output spectra, i.e., resolution exceeding 5000 where resolution is defined as the ratio of the number of mass units of the mass line to the width of the mass line in mass units at its half amplitude height. However, such high resolution was achieved by a spectrometer with an ion analyzer design having on the order of 60 separate electric analyzer electrodes to form the uniform electric field. These electrodes are manufactured to a high degree of precision and are therefore expensive to produce. The 60 analyzer electrodes were far more than the minimum number required to produce the same resolution had the magnetic field been properly homogenized.

It turns out that the maximum mass resolution obtainable from a cycloidal spectrometer is equal to the homogeneity of the electric field and one half the homogeneity of the magnetic field. Thus in order to obtain a mass resolution of 5000 the electric field E must be uniform, over the ion trajectory, to at least one part in 5000 and similarly the magnetic field must be uniform to at least one part in 10,000.

In the present invention, ion analyzer design criteria are established which permit using the minimum number of electric field producing electrodes to achieve optimum mass resolution of the output of a cycloidal mass spectrometer employing a magnetic focusing field of a certain homogeneity. Using this design criterion the electric field 3,437,805 Patented Apr. 8, 1969 ice ion focusing structure may be simplified to reduce spectrometer manufacturing costs.

The principal object of the present invention is the provision of an improved cycloidal mass spectrometer offering ease of manufacture for a given mass spectra resolution.

One feature of the present invention is the provision of an optimum ion analyzer design having the least number of electric field producing electrodes for a certain magnetic field homogeneity, whereby spectrometer construction costs are minimized.

Other features and advantages of the preesnt invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a side elevational view, partly in section and partly schematic, of a cycloidal mass spectrometer employing the features of the present invention,

FIG. 2 is a graph of optimum width W to spacing S ratio versus optimum obtainable resolution for an electrode geometry of an ion analyzer section of the spectrometer of FIG. 1.

FIG. 3 is a sectional view of a portion of the structure of FIG. 1 taken along line 33 in the direction of the arrows, and

FIG. 4 is a sectional view of a portion of the structure of FIG. 1 taken along line 44 in the direction of the arrows.

Referring now to FIG. 1 there is shown a cycloidal mass spectrometer system. More particularly, an array of generally rectangular shaped ring electrodes 1 are insulatively supported within a thin rectangular vacuum envelope 2, only partially shown, from a heavy rectangular flange, not shown, which closes off one end of the vacuum envelope.

The separate rings 1 of the electrode array are operated at slightly different electric potentials derived from a voltage source 3 via leads 4 connected at nodes 5 of a voltage divider network 60. The different potentials applied to the difierent rings 1 establishes a region of uniform electric field E in the hollow interior of the ring electrode array. The electric field E is directed parallel to the line of development of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field H directed at right angles to the direction of the electric field E. The field H is conveniently produced by an electromagnet 7 with the vacuum envelope 2 being disposed in the narrow gap defined between a pair of pole pieces 8 of the magnet 7.

The envelope 2 is evacuated in use via pump 10 to a suitably low pressure as of 10* torr. Gas to be analyzed by the analyzer section, including the array of electrodes 1, is introduced from a source 9 into the analyzer section through the vacuum envelope 2 via an inlet tubing 11 as of stainless steel. The inlet tubing 11 feeds gas at a desired rate into an ion source 12. The ion source ionizes the gas and projects it through a slot into the crossed magnetic field H and electric field E of the analyzer.

Under the influence of the crossed electric and magnetic fields the ions are caused to execute cycloidal trajectories. However, only ions of a certain mass number, for a given intensity of E and H, will be focused at a detector slot 13 a certain focal distance from the source and at the same electric potential. An ion detector 14 is positioned behind the slot 13 to produce an output corresponding to the number of ions under analysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signal and feeds it to the Y axis of an X-Y recorder 16 wherein it is recorded as a function of a scan of the magnetic field intensity H produced by a scan generator 17. The output of the recorder 16 is a mass spectrum of the sample under analysis.

The resolution of the recorded mass spectrum, as previously defined, is limited by the homogeneity of both the electric and magnetic focusing fields over the cycloidal trajectories of the detected ions. The detected mass resolution can be no better than the homogeneity of the region of the electric field E traversed by the collected ions, where the homogeneity of the electric field is the inverse of the ratio of the characteristic amplitude of the departure of the peaks and valleys of the electric field from a median value of the field over the region of interest. Likewise the detected mass resolution can be no better than one half the homogeneity of the region of the magnetic field H traversed by the detected ions, where magnetic field homogeneity is defined in the same manner as the homogeneity of the electric field E.

It has been found that for a certain electric field homogeneity and thus certain obtainable resolution there is a certain optimum ratio of ring width W, as defined in FIG. 3, to ring spacing S, as defined in FIG. 4. This optimum ratio of W/S determines the minimum number of rings that will be required for a certain mass output resolution and therefore is very useful for reducing fabrication costs of the ring electrodes 1 as Well as reducing the cost of associated ring energizing circuitry.

FIG. 2 shows a plot of W/S as a function of maximum obtainable resolution and required magnetic field homogeneity to achieve the optimum resolution. The plot of FIG. 2 assumes the ion trajectories will occupy plus and minus of W as measured from the centerline 21.

In a preferred embodiment, the array of rings 1 has a relatively low transparency from the side in order to prevent stray electric fields from entering the interior region of the ring array such that the width W may be reduced to reduce the magnetic field requirements. Transparency is defined as the ratio of the spacing D between adjacent portions of the rings 1 to the spacing S between centers of the rings. Suitable transparencies are 25% or less. In addition, the rings 1 should have a thickness 1 which is greater than the spacing D between rings 1 and preferably at least twice the spacing D.

The graph of FIG. 2 is used as follows to obtain an optimum analyzer design: A certain desired resolution is chosen. For this resolution the magnetic field homogeneity would have to be at least as good as shown on the bottom scale. The ratio of W/S is read, for this resolution, from the graph. The width W of the rings 1 is chosen to be compatible with the space available in the magnet 7 producing the field. Once W is chosen then the spacing S is determined. The ion trajectories are determined by the ion optics of the spectrometer including the focal distance between source and detector slots. An equation which defines, and which is determinable from, the plot of FIG. 2

S 3 Equationl where R is the mass resolution and W and S are defined above. Thus, the ratio of W/S may equally as well be determined from Equation 1 as from the graph of FIG. 2.

In a typical example of a cycloidal mass spectrometer using the features of the present invention, the rings 1 had a width of about 1" and the long rings had a length of 7 inches and the short rings a length of 3.25" with a spacing S between rings 1 of approximately 0.300".

The rings 1 had a height h of 0.250", a separation D of about 0.050", and a thickness t of 0.250. The rings 1 were made of stainless steel and ground flat on their adjacent sides to $0.001".

Sixteen rings 1 were used to contain ion beam trajecto- Log R ries which had a maximum diameter of about 5.5" centered in the gap of an electromagnet having 9" diameter pole pieces 8. In this design, mass resolution of 6000 has been observed Which is approximately at the theoretical limit of the design.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high resolution cycloidal mass spectrometer apparatus including, means forming an array of spaced electrodes for producing an ion focusing electric field at right angles to a DC. magnetic field in which said array of electrodes is to be immersed in use, means forming a source of ions and for projecting the ions into the electric field to cause the ions under the influence of the combined electric and magnetic fields to traverse cycloidal trajectories, means at the end of the cycloidal ion trajectories for detecting the ions and for deriving an output spectrum having a certain mass resolution R, said electric field producing electrodes each having portions disposed on opposite sides of the cycloidal ion trajectories with the distance between said opposed sides being defined as W, said electrodes being spaced apart on their center by a distance S taken in the direction of the line of development of said array and at right angles to the direction along which W is measured, and wherein said electrodes have characteristic W to S ratios within L25 of (that optimum) the value according to the (graph) formula:

where R is the mass resolution of the derived output spectrum, whereby the number of electric field electrodes is minimized for a certain maximum attainable mass resolution.

2. The apparatus according to claim 1 wherein R is greater than 1000 and less than 100,000.

3. The apparatus according to claim 1 wherein R is greater than 5000 and less than 100,000.

4. The apparatus according to claim 1 including, means for producing the magnetic focusing field in which said electrode array is immersed, said magnetic focusing field having a certain characteristic homogeneity over the ion trajectories falling within the range of i-50% of 2R.

5. The apparatus according to claim 4 wherein said magnetic focusing field has a characteristic homogeneity greater than 2000 and less than 200,000 over the detected ion trajectories.

6. The apparatus according to claim 1 wherein said electrodes are ring-shaped with individual heights h greater than the spacing D between adjacent rings.

7. The apparatus according to claim 6 wherein the ratio of the spacing between said rings D to the center line spacing S of said rings is less than 25%.

S. The apparatus according to claim 5 wherein the magnetic focusing field has a homogeneity greater than 20,000 and less than 200,000.

Log R References Cited UNITED STATES PATENTS 2,221,467 11/1940 Bleakney 25041.9 2,946,887 7/1960 Castle 250--41.9

RALPH G. NILSON, Primary Examiner.

S. C. SHEAR, A s stant Examiner. 

